Conventionally, such a type of flowmeter is known, for example, in Japanese Laid-Open Publication No. 9-15006. As shown in FIG. 64, the flowmeter includes: a sampling program 2 for reading a measurement value, at an interval having a predetermined first sampling time, from an analog flow sensor 1 that measures the flow rate of gas; a consumed gas amount calculation program 3 for calculating the flow rate of consumed gas at a predetermined time; a mean value calculation program 4 for calculating the mean value of measurement values, which are read from the analog flow sensor at the first sampling time, at an interval of a second sampling time within a predetermined time period; a pressure variation frequency estimation program 5 for estimating the frequency of a pressure variation based on an output of the flow sensor; and a RAM 6 which functions as a memory. Herein, reference numeral 7a denotes a CPU for executing the programs, and reference numeral 7b denotes a ROM for storing the programs. In such a structure, a measurement process is performed such that the predetermined measurement time is equal to or longer than a single cycle of the vibration frequency of a pump or is a multiple of the cycle. Averaging is performed to suppress variation in the flow rate.
As another conventional example, the invention disclosed in Japanese Laid-Open Publication No. 10-197303 is known. As shown in FIG. 65, the flowmeter includes: flow rate detection means 8 for detecting the flow rate; frequency detection means 9 for detecting the frequency of a variation of a flow; and measurement time set means 10 for setting the measurement time for flow rate detection to about a multiple of one cycle of the variation frequency. Herein, reference numeral 11 denotes flow rate calculation means; 12 denotes measurement start means; 13 denotes signal processing means; and 14 denotes a flow rate. With this structure, the flow rate is measured in accordance with the frequency of a variation waveform, whereby a correct flow rate measurement is achieved within a short time period.
As still another conventional example, the invention disclosed in Japanese Laid-Open Publication No. 11-44563 is known. As shown in FIG. 66, the flowmeter includes: flow rate detection means 15 for detecting the flow rate; variation detection means 16 for detecting a variation waveform of the flow rate of fluid; pulse measurement means 17 for starting the measurement of the flow rate detection means when an alternating component of the variation waveform is in the vicinity of zero; and flow rate calculation means 18 for processing a signal from the flow rate detection means. Herein, reference numeral 19 denotes a signal processing circuit; 20 denotes a time measurement circuit; 21 denotes a trigger circuit; 22 denotes a transmission circuit; 23 denotes a comparison circuit; 24 denotes an amplification circuit; 25 denotes a switch; 26 denotes a measurement start signal circuit; and 27 denotes start-up means; 28 denotes a flow path. In this structure, the flow rate near the average of the variation waveforms is measured, whereby a correct flow rate measurement is achieved within a short time period.
As yet another conventional example, the invention disclosed in Japanese Laid-Open Publication No. 8-271313 is known. As shown in FIG. 67, whether or not a flow rate value has been detected in flow sensor measurement (29) is confirmed (30). Until a flow rate is confirmed to have been detected, the process does not proceed, and the measurement with the flow sensor is continued. Once a flow rate is found, it is determined whether or not the flow rate Q is equal to or higher than a predetermined value (31). When the flow rate Q is equal to or higher than the predetermined value, it is further determined whether or not the pressure variation surpasses a predetermined value Cf (32). When the pressure variation does not surpass a predetermined value Cf, measurement 34 is performed with a piezoelectric film sensor of a fluidic flowmeter. When the pressure variation surpasses a predetermined value Cf, it is confirmed if the pressure variation surpasses a second predetermined value (33). When the pressure variation surpasses the second predetermined value, the measurement (34) is performed with the piezoelectric film sensor of the fluidic flowmeter. When the pressure variation does not surpass the second predetermined value, the measurement (29) is performed with the flow sensor.
As shown in FIG. 68, ultrasonic wave transducers 51 and 52 are provided in a flow rate measurement section 50 so as to oppose the direction of a flow. A control section 53 starts a timer 54, and simultaneously, outputs a transmission signal to a driver circuit 55. An ultrasonic wave is transmitted from the ultrasonic wave transducer 51 which received an output of the driver circuit 55. The ultrasonic wave is received by the ultrasonic wave transducer 52. A reception detection circuit 56 which received an output of the ultrasonic wave transducer 52 detects the ultrasonic wave and stops the timer 54. By such an operation, a time (t1) spent from a time when an ultrasonic wave is transmitted from the ultrasonic wave transducer 51 to a time when the wave is detected by the ultrasonic wave transducer 52 is measured. Next, a switching circuit 58 is operated based on a signal from the control section 53, such that the driver circuit 55 and the ultrasonic wave transducer 52 are connected, and the reception detection circuit 56 and the ultrasonic wave transducer 51 are connected. Under this state, transmission and reception of an ultrasonic wave is performed again to measure a time (t2) spent from a time when an ultrasonic wave is transmitted from the ultrasonic wave transducer 52 to a time when the wave is detected by the ultrasonic wave transducer 51. Based on the two propagation times (t1) and (t2), a calculation section 57 calculates the flow rate from a difference between inverse numbers of the propagation times.
As a conventional example of this type of flowmeter, the invention disclosed in Japanese Laid-Open Publication No. 6-269528 is known.
However, in the first of the above conventional inventions, the gas flow rate is measured by using a mean value. Therefore, measurement over a long time period is necessary in order to obtain a reliable mean value, and hence such flow rate measurement cannot be performed within a very short space of time. In the second of the above conventional inventions, measurement cannot deal with a variation in frequency. In the third and fourth conventional inventions, the method for measuring the flow rate must be changed according to the presence/absence of a pressure variation, and it is necessary to provide two means, pressure measurement means and flow rate measurement means. In the first to forth inventions, when any abnormality occurs, measurement either cannot be performed, or can be performed but with decreased accuracy.
Still further, in the above conventional structures, when receiving a signal, if noise which is in synchronization with the measurement frequency or transmission frequency of an ultrasonic wave is present, the noise is superposed on the signal always at the same phase when the propagation time is the same. The noise is counted as a measurement error, and accordingly, correct measurement cannot be performed. Moreover, when the propagation time is varied due to a variation in temperature or the like, the phase at which noise is superposed is varied, and accordingly, a measurement error is varied. As a result, a correction value cannot be stabilized. Furthermore, since the measurement resolution is determined based on the resolution of the timer 54, simply averaging the measurement values cannot increase the accuracy of measurement. Thus, it is necessary to increase the resolution of the timer 54 in order to perform measurement which requires the resolution. When the operation clock of the timer 54 is increased so as to have a high frequency, various problems occur, i.e., an increase in current consumption, an increase in high-frequency noise, and an increase in size of circuitry. Thus, there exists an objective to increase the resolution of measurement with a timer which operates at a low frequency in order to increase the measurement accuracy.
In the fifth conventional invention, a delay means is inserted between a control section and a drive circuit, and the amount of delay is changed such that a reflected wave is avoided. In this way, an effect by the reflected wave is reduced. For example, the ultrasonic wave transducer at a receiving side is vibrated due to noise generated when the ultrasonic wave is transmitted. Thus, a variation in the signal-reception detecting time, which is caused by superposition of reverberation of this vibration on the ultrasonic wave signal, cannot be decreased.
The present invention seeks to solve the above problems. A first objective of the present invention is to set an optimum number of times that the measurement is repeated according to a variation of a flow by detecting a variation frequency using software but without using additional variation detecting device, and successively changing the number of repetition times. Further, it is sought to achieve a measurement flow rate in a reliable and accurate manner within a very short space of time even when there is a change in pressure variation and variation frequency. A second objective of the present invention is to instantaneously perform highly accurate flow-rate measurements by switching so as to detect a variation with transmission/reception means without using an additional variation detecting device and performing measurement processing in synchronization with a variation. A third objective of the present invention is to perform highly accurate flow-rate measurement, even when any abnormality occurs in the measurement process, by quickly detecting the abnormality with measurement monitoring means and appropriately processing the measurement. A fourth objective of the present invention is to perform flow-rate measurement in a reliable and accurate manner within a very short space of time by using instantaneous flow rate measurement means and digital filter means. A fifth objective of the present invention is to measure a flow rate value with a high accuracy even when there is a variation in temperature.